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Though I’m fonder of snakes and cats now, hamsters were the pet of my childhood. Hamsters, like most rodents, are nocturnal. Unfortunately for me, I was not. I was a light sleeper as a child and remain one to this day. When I was trying to fall asleep before another big day in elementary school, I’d often be kept awake by the persistent squeaking of the hamster wheel. Aside from food hoarding and nest building, it really did seem like running on the wheel was one of my hamsters’ favorite activities.

Years later, after I’d long since given up my hamster habit, I thought about their noisy nighttime running. Had my pets really been enjoying themselves, or were they running because they were driven to neuroses by the boredom of captivity? As it turns out, I’m not the only one with this question.

In 2014, Johanna Meijer and Yuri Robbers set up an experiment. They decided to place running wheels in the wild to see if rodents would still run on them even if they had a world of other things to do. They set up wheels in two different locations where wild rodents frequented. One was placed in a green urban area, while another went in an isolated dune environment. This way, they could see if human presence might have an effect on running wheel use. They put food at each station as well, and motion-detecting cameras to record any critters that happened by.

For three years they monitored the sites, examining thousands of recordings to see which animals were using the wheel. What they found was surprising. In the first two years, the running wheels were used a combined 1,011 times. Of those, 734 were mice. Shrews and rats also made appearances, but slugs and frogs also “ran” on the wheel. Snails oozed over the wheel around on camera, but they just seemed to be randomly crawling over the wheel, not displaying “proper wheel running” like their slug brethren.

Though the slugs appeared to have ended up on the wheel accidentally, mice, shrews, rats, and frogs were more deliberate. Some would leave the wheel and return again after a few minutes to run another round. Of the mice that came by, 1.7% would try running on the wheel. This number was only 0.4% when all animals were included, so it’s apparently mice that have a real passion for jogging.

Are they just hungry?

After some time, the researchers wondered whether the food they put out might be skewing their results. Maybe the animals liked the wheel just because they associated it with an easy meal. Indeed, once they took the food away, fewer animals came by the research area. However, the ones that did visit were four times more likely to try out the wheel than before. Something about running on a wheel was attractive to those animals, even without a calorie boost to go with it.

What does this say about pet rodents?

When Meijer and Robbers compared the amount of running by wild mice vs captive ones, they found that both groups spent approximately the same amount of time on the wheel per running session. Obsessive wheel running is still probably an unhealthy trait, but moderate use doesn’t appear to be a sign of mental duress in mice. It could be that mice find running stimulating, or maybe it’s some form of play. The motivation behind exercise wheel use may remain a mystery, but it can now be considered a natural behavior. I’m just glad these mice weren’t keeping me up at night!